Transparent, abrasion resistant coating compositions

ABSTRACT

There is disclosed transparent, abrasion resistant coating compositions comprising a colloidal dispersion of a water insoluble dispersant in a water-alcohol solution of the partial condensate of silanol wherein the dispersant comprises metals, alloys and salts thereof.

This is a continuation of application Ser. No. 845,915, filed 10-27-77and now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to coating compositions and in particular,relates to protective coating compositions for polymeric materials.

In recent year, synthetic polymeric materials have been utilized in awide variety of applications. Transparent polymeric materials have beenutilized as windows in aircraft and public buildings. Further,transparent polycarbonates and acrylics have been utilized in glazingfor automobiles, buses and aircraft. Although these transparent polymersmay be readily fabricated into the desired shape, unfortunately theyhave relatively low abrasion resistance. The prior method of enablingthe use of these transparent polymers involves coating the surface ofthe polymers with a transparent, abrasion resistant coating. Priorcoating such as silica-containing solutions and polysilicic acidfluorinated copolymer have been utilized. Unfortunately, these coatingsare difficult to apply, have a low humidity resistance or are expensive.

Another prior coating composition is disclosed in the Clark U.S. Pat.No. 3,986,997. The Clark composition comprised a colloidal dispersion ofsilica in a water-alcohol solution of the partial condensate of silanol.Although the Clark composition is suitable in some applications, inother application, additional hardness, elongation and staticdissipation was desired to enable the formation of a more suitablecoating. Thus, a coating having an improved hardness, elongation andstatic dissipating capability is still desired.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved abrasion resistant coating composition. It is a further objectof the present invention to provide a coating composition havingimproved elongation and static dissipating capabilites.

This and other objects and advantages are obtained by forming atransparent, abrasion resistant coating comprising a colloidaldispersion of a water insoluble dispersant in a water-alcohol solutionof the partial condensate of silanol. The insoluble dispersant ispreferably a metal, metal alloy or metal salt that can be converted intoa colloidal dispersion and is stable in the pH range of about 3 to 6.The silanol preferably has the formula R(Si(OH)₃) where R is an organicradical such as a lower alkyl or vinyl, methoxyethyl, phenyl,γ-glycidoxy propyl or γ-methacryloxypropyl radical. Preferably, thepartial condensate contains at least 30% by weight methyl silanol.

A more thorough disclosure of the objects and the advantages of thepresent invention is presented in the detailed description whichfollows.

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates the formation of a transparent,abrasion resistant coating comprising a colloidal dispersion of a waterinsoluble dispersant in a water-alcohol solution of the partialcondensate of silanol.

The insoluble dispersants are preferably a metal, metal alloy or metalsalt which can be dispersed in the water alcohol solution of the partialcondensate of silanol to form a colloidal dispersion. Suitable metalsare antimony, aluminum, gold, silver, copper, tin, cadmium, and indium.Suitable alloys are mixtures of the above metals such as tin-antimony,tin-indium and tin-cadmium alloys. Suitable metal salts are oxides,nitrides, phosphates, sulfides, hydroxides or acetates of the abovemetals and alloys such as antimony oxide, aluminum oxide, aluminumacetate, tin-antimony oxides, cadmium stannate and indium-tin oxide.However, it will be obvious to one skilled in the art that other waterinsoluble metals, metal alloys and metal salts which can be dispersed ina water-alcohol solution of the partial condensate of silanol to form acolloidal dispersion, are stable in the pH range of about 3 to about 6,and do not alter the silicone cure reaction or coating stability wouldalso be suitable in the practice of the present invention.

In some cases, the dispersants of the present invention may be opaque.Suitable opaque dispersants preferably have a particle size small enoughto avoid appreciable scattering of visible light thereby preventing theformation of a translucent or pigmented coating. Preferably, if thedispersant has a refractive index of about 1.5, the average colloidalparticle size should be about 200 angstroms, whereas if the refractiveindex is 2.0 or greater, the average colloidal particle size should beabout 100 angstroms or less.

The colloidal dispersants function to improve the hardness and theelongation properties of the coating. Further, certain colloidal metals,metal alloys and metal salts such as copper, gold, silver, tin-antimonyoxides, cadmium stannate and indium tin oxide also provide improvedstatic dissipating properties.

The partial consensate preferably comprises trisilanols having theformula R(Si(OH)₃) wherein R is a low molecular weight, short chainorganic radical and is preferably selected from the group of radicalsconsisting of lower alkyl or vinyl (C₁ -C₄), methoxyethyl, phenyl,γ-glycidoxy propyl or γ-methacryloxypropyl. The organic radicalfunctions to enable the formation of the polymeric coating. However,larger organic radicals hinder the crosslinking of the silanol andresult in a softer coating. The partial condensate is preferablycomprised of at least about 30% by weight methyl trisilanol and mostpreferably 100% by weight methyl trisilanol.

The partial condensate may also comprise mixtures of trisilanols anddisilanols. The partial condensate comprising a mixture of tri anddisilanols forms coatings having increased flexibility and decreasedhardness. Therefore, it is preferred that the mixture comprise at leastabout 80% by weight of the trisilanol to form a coating having asuitable hardness.

The silanols are preferably generated in situ by the hydrolysis of thecorresponding alkoxysilane in an aqueous dispersion of the insolubledispersant. Suitable alkoxysilanes have alkoxy substituents such asmethoxy, ethoxy, isopropoxy, methoxyethoxy t-butoxy and acetoxy which,upon the hydrolysis of the silane, form the corresponding alcohol oracid. Upon the formation of the silanol in the acidic aqueous medium, aportion of the silicon-bonded hydroxyl groups of the silanol arecondensed to form silicon-oxygen-silicon bonds. However, the silanol isonly partially condensed and retains a portion of the silicon-bondedhydroxyl groups to render the polymer soluble in the water-alcoholsolvent. The partial condensate can be characterized as having at leastone silicone-bonded hydroxyl group for every threesilicon-oxygen-silicon bonds. When the coating is cured, the remaininghydroxyl groups condense to form a selsesquioxane, RSiO₃ /₂.

The coating composition of the present invention is preferably preparedby dispersing the dispersant in an aqueous or aqueous-alcohol solutionto form a hydrosol. Suitable alcohols are lower aliphatic alcohols suchas methanol, ethanol, propanol and isopropanol. However, it will beobvious to one skilled in the art that other alcohols may also beutilized in the practice of the present invention. The hydrosol is thenadded to a solution of the alkoxysilane in acid. The solution ofalkoxysilane contains a sufficient amount of acid so that the resultingmixture preferably has a pH from about 3 to about 6. At lower or higherpH, the dispersant tends to precipitate out from solution. The acid ispreferably an organic acid such as acetic, chloroacetic, formic, anddimethylmalonic because they evaporate readily when the coating is driedand they enhance adhesion of the coating to the substrate. Othersuitable organic and inorganic acids are disclosed in the Clark U.S.patent, supra, the disclosure of which is incorporated herein byreference.

In a short time after the hydrosol and alkoxysilane solution are mixedtogether, substantially all of the alkoxysilane is hydrolyzed to formthe corresponding silanol and alcohol. A portion of the silanol is thencondensed to form the partial condensate. It is preferred that themixture contain enough alcohol to form a water-alcohol co-solvent systemhaving from about 20% to about 75% by weight alcohol to insure thesolubility of the partial condensate. Additional alcohol or anotherpolar solvent such as acetone may be added to the mixture to form asuitable solvent system.

Preferably, the coating composition contains from about 10% to about 50%solids and the solids consist of about 10% to about 70% of the colloidaldispersant, and about 30% to about 90% of the partial condensate.However, it will be appreciated by those skilled in the art that theamount of solids in the coating and the composition of the solids mayvary over a wide range depending upon the desired properties of thecoating.

Preferably, a latent condensation catalyst is added to the coatingcomposition to enable curing of the coating composition under milderconditions to form the hard, abrasion resistant coating. Preferably asufficient amount of the catalyst is added to the composition tocomprise about 0.1% to about 1% by weight of the composition. Suitablecondensation catalysts for the present invention are choline acetate andsodium acetate. Other suitable condensation catalysts are disclosed inthe Clark patent.

The coating composition of the present invention has stable shelf lifeof about six months to one year. The composition can be readily appliedto a variety of substrates such as glass, polycarbonates, acrylics, etc.by methods well known to the art skilled such as flow, spray or dipcoating. Then the coating composition is cured at temperatures of about65° to about 130° depending on the stability of the substrate for aperiod of time from about 1 to about 24 hours depending on thetemperature to complete the condensation reaction and form thetransparent abrasion resistant coating of the present invention. Thecoating preferably has a thickness of about 1 to about 20 microns.

The following examples illustrate the present invention. The physicalproperties of the coating compositions of the present invention werecompared to the physical properties of a coating formed according to theteachings of the Clark patent. The compositions of the present inventionwere formed as follows

Composition I

An antimony oxide colloidal dispersion in methyl silanol was made byreacting 18.0 grams of the oxide with a mixture of 18.0 grams oftrimethoxy methyl silane and 1.3 grams of glacial acetic acid. Thetemperature was maintained from about 0° C. to 10° C. 31.6 grams of2-propanol was then added to the solution. The mixture was allowed todigest for a period of about 48 hours and then 16.0 grams of 2-propanol,12.6 grams of butanol and 0.9 grams of a 10% choline acetate solution in2-propanol was added to the mixture. The mixture was then filtered andcoated onto an acrylic substrate by flow film application. The coatingwas then dried to a tack-free condition at about 40% humidity at about20° C. and cured at about 85° C. to 95° C. for about two hours.

Composition II

A dispersion comprising 208 grams of basic aluminum acetate in 312 gramsdistilled water was added to 2000 grams of isopropyl alcohol withconstant stirring at a temperature of about 5° C. to 10° C. Then, 250grams of methyl trimethoxy silane was added slowly with stirring to themixture while maintaining the reaction temperature. After one hour ofstirring, 142 grams of isopropyl alcohol and 0.9 grams of a 10% cholineacetate solution in 2-propanol was added to the mixture and the mixturewas stirred for another hour. The product was filtered through a 1.2micron filter using diatomaceous earth as a filtering aid. Thecomposition was coated on acrylic as 26% solid by flow film application.The coating was then dried to a tack free condition at about 40%humidity at about 20° C. and cured for four hours at 87° C.

The coated parts were tested for light transmission and haze using ASTMC-1003; taber abrasion using ASTM D-1044 (500 gms, 100 rev.), andadhesion and Elongation-Flexibility as follows:

Adhesion:

Cross-hatch scribes are made on the surface of samples of the coatedparts and 3M-600 cellophane tape is then affixed to the surface. Thetape is then pulled away from the surface to remove the coating. Thenumber of unaffected squares determines the percentage of adhesion ofthe coating to the substrate.

Elongation-Flexibility:

Samples of the coated material having dimensions of one inch by teninches are wrapped around mandrels having decreasing diameters from 84inches to 8 inches. Each sample is then inspected with high intensitylamps to determine coating failure as evidenced by perpendicularcracking across the coating.

The Percent Elongation is then calculated as follows: ##EQU1##

The results of the test are as follows:

    ______________________________________                                                               Compo-   Compo-                                                        Clark  sition   sition                                                        Coating                                                                              I        II                                            ______________________________________                                        Light Transmission, %                                                                           93.9     92.0     93.2                                      Haze, %           0.2       0.08     0.25                                     Taber Abrasion, % delta haze                                                                    2.5      0.7      1.0                                       Flexed Elongation 1.0      1.5      1.0                                       Adhesion %        100      100      100                                       ______________________________________                                    

From the above data, it can be seen that the coating composition of thepresent invention exhibits improved taber abrasion resistance andelongation. Further, the coating exhibits suitable adhesion to thesubstrates.

While embodiments and applications of this invention have been shown anddescribed, it will be apparent to those skilled in the art that manymore modifications are possible without departing from the inventiveconcepts herein described. The invention, therefore, is not to berestricted except as is necessary by the prior art and by the spirit ofthe appended claims.

We claim:
 1. A coating composition which forms a transparent,abrasion-resistant coating upon curing, said coating compositionconsisting essentially of an effective abrasion-resisting amount of acolloidal dispersion of a water insoluble dispersant in a water-alcoholsolution of the partial condensate of R Si (OH)₃ where R is an organicradical, and said dispersant is a metal or a metal alloy selected fromthe group consisting of antimony, aluminum, gold, silver, copper, tin,cadmium, indium and mixtures thereof.
 2. The coating composition ofclaim 1 wherein said organic radical is selected from the groupconsisting of C₁ -C₄ alkyl and vinyl, methoxyethyl, γ-glycidoxypropyland γ-methacryloxypropyl.
 3. The coating composition of claim 1 whereinsaid partial condensate contains at least 30 percent by weight methyltrisilanol.
 4. The coating composition of claim 1 wherein said partialcondensate contains 100 percent by weight methyl trisilanol.
 5. Thecoating composition of claim 1 wherein said composition additionallyincludes a latent condensation catalyst.
 6. The coating composition ofclaim 5 wherein said catalyst is choline acetate.
 7. The coatingcomposition of claim 1 wherein the alcohol in the water-alcohol solutionis a lower aliphatic alcohol.
 8. A coating composition which forms atransparent, abrasion-resistant coating upon curing, said coatingcomposition consisting essentially of an effective abrasion resistingamount of a colloidal dispersion of a water insoluble dispersant in awater-alcohol solution of the partial condensate of a mixture of RSi(OH)₃ and R₂ Si(OH)₂ wherein R is an organic radical, and saiddispersant is a metal or a metal alloy selected from the groupconsisting of antimony, aluminum, gold, silver, copper, tin, cadmium,indium and mixtures thereof.
 9. The coating composition of claim 1wherein said dispersant has a partial size of less than about 200angstroms.
 10. The coating composition of claim 1, wherein said partialcondensate contains at least 30 percent by weight of CH₃ Si (OH)₃. 11.The coating composition of claim 1 wherein said dispersant is antimonyor an alloy containing antimony.
 12. The coating composition of claim 1wherein said dispersant is aluminum or an alloy containing aluminum. 13.The coating composition of claim 1 wherein said dispersant is gold or analloy containing gold.
 14. The coating composition of claim 1 whereinsaid dispersant is silver or an alloy containing silver.
 15. The coatingcomposition of claim 1 wherein said dispersant is copper or an alloycontaining copper.
 16. The coating composition of claim 1 wherein saiddispersant is tin or an alloy containing tin.
 17. The coatingcomposition of claim 1 wherein said dispersant is cadmium or an alloycontaining cadmium.
 18. The coating composition of claim 1 wherein saiddispersant is indium or an alloy containing indium.